The immediate-type hypersensitivity, such as extrinsic asthma, hay fever, and allergic responses to certain food or drugs, is mediated primarily by immunoglobulin E (IgE). In an IgE-mediated allergic response, the allergen binds to IgE on the surface of mast cells and basophilic leukocytes (basophils). This binding causes a crosslinking of the IgE molecules and hence the underlying receptors for the Fc portion of IgE (Fc.epsilon.R) and thereby triggers the release of pharmacologic mediators, such as histamine, slow-reacting substance of anaphylaxis and serotonin. The release of these mast cell and basophil products causes the various pathological manifestations of allergy.
IgE is produced by surface IgE-bearing B lymphocytes (B cells). In individuals sensitized to specific allergens, the allergen-specific IgE is produced by B cells continuously. Recently, Whitaker (U.S. Pat. No. 4.714.759) described a method of treating allergy patients with toxin-conjugated antibodies that were specific for IgE isotype. The intended pharmacological effects of the immunotoxin is to kill IgE-producing B cells.
In the U.S. patent application Ser. No. 140,036 (now abandoned), filed Dec. 31, 1987 and in its Continuation-in-Part application Ser. No. 07/226,421, filed Jul. 29, 1988, it was noted that IgE bound to the receptors for the Fc of IgE (Fc.epsilon.R) on the surface of basophils and mast cells very strongly. The association constant, Ka, is in the neighborhood of 1.times.10.sup.10 liter/mole and the "off" time is more than 20 hour. Even though IgE is not synthesized by basophils and mast cells, the very strong and stable association of IgE with Fc.epsilon.R makes IgE virtually a surface antigen of these cells. It was indicated in our applications that an immunotherapeutical agent targeting the IgE on B cells must not react with the IgE on basophils and mast cells. Antibodies which react with IgE isotype will cross-link IgE and the underlying Fc.epsilon.R on basophils and mast cells and, when administered in vivo, will induce systematic histamine release leading to anaphylaxis.
In the related patent applications mentioned above the development of monoclonal antibodies that recognized an antigenic epitope present on the IgE on B cells but not on the IgE on basophils was described. In addition, the method of using the antibodies for treating allergy, either in the form of plain antibodies or toxin-conjugated antibodies was described. The plain antibodies can cause the pharmacological mechanism of antibody-dependent cellular cytotoxicity (ADCC) and the toxin-conjugated antibodies can directly cause cytolysis. Both of these mechanisms can lead to the depletion of IgE-bearing B cells without harming the basophils and mast cells. The IgE epitopes present on B cells but absent on basophils were termed ige.bl epitopes (bl stands for B lymphocytes).